Fix options parsing to support -d option properly

Provides a more convenient method to specify multiple dirs instead of
in a comma-separated list.  For example, user can indicate multiple dirs
this way:

  $ rerun -d example_dir -d ../tmp/dir "rake spec"

Also add some unit tests to support new options.
Merge pull request #37 from bsia/fix-multidir-opt

An alternative (hopefully better) way to specify multiple dirs (by bsia)